Primary batteries are batteries designed to be used once and discarded. Secondary batteries are rechargeable and typically can be used over many charge-discharge cycles. In many circuits a plurality of primary or secondary batteries are used. Some circuits are designed to use a plurality of separate battery cells (primary cells or secondary cells) connected in series to provide a higher voltage collectively than a single cell provides. Other circuits are designed to be used with a plurality of separate battery cells (primary cells or secondary cells) connected in parallel to provide a higher current and capacity (e.g., mAh) collectively than a single cell can deliver.
In typical series configurations, the plurality of separate battery cells can simply be connected in series circuit communication without additional circuitry. Typical parallel battery designs do not connect the plurality of separate battery cells in parallel without additional circuitry because at least one battery will be a drain on the other. More specifically, in theory, when batteries are placed in parallel, the capacities of each cell add because a fraction of the current that goes to the device comes from each cell. In the experience of the Applicant, however, when batteries are placed in parallel, their capacities do not add. On paper the math works, but in practice the capacities of each cell are actually reduced. Because the cells in parallel are not perfectly balanced, in two-cell parallel circuits, the stronger cell will charge the weaker cell. Hysteresis effects of the battery's terminal voltage allow the cell being charged to now become the stronger of the two, and the once weaker cell will start to charge the cell that was doing the charging earlier. This back and forth process typically continues until the two batteries have prematurely completely discharged each other.
What is typically done to prevent parallel batteries from charging each other is to put diodes in series with the batteries and the series battery/diode combinations are connected in parallel, as shown in FIG. 1. The configuration of FIG. 1 works for many applications, but not when the battery cell voltage is near the operating voltage of the load. This is because the forward voltage drop of the diodes is substantial, e.g., about 0.6 volts for standard rectifier diodes, and around 0.3 volts for Schottky diodes. This voltage drop is too high for some circuits, and the electronics will not work properly because the lower limit of operating voltage has been reached or exceeded.
Lithium primary cells serve as an example. Many variations of primary lithium chemistry cells exist, but nearly all of the batteries have a terminal voltage close to 3 volts. This is convenient because most modern circuitry is designed to operate with a 3 volt supply. The CR123A battery is readily available and relatively inexpensive and has a useable capacity near 1100 mAh. However, this capacity is much less than even a small alkaline battery such as the AA battery, which typically has a capacity of about 2500 mAh. Alkaline batteries do not provide the performance required over an extended temperature range that is required in an industrial or commercial application, especially cold temperatures. The CR123A lithium manganese dioxide cell offers high pulse current capability over an extended temperature range. With the lower capacity, it would be helpful to be able to couple a plurality of lithium cells in parallel; however, using diodes is unacceptable because the 0.3 to 0.6 volt voltage drop is too high, and the 3 volt electronics will not work properly because the lower limit of operating voltage has been exceeded.